1. Field of Invention
The present disclosure relates to charged particle sources and, more particularly, to a charged particle source created by photoionizing an atom beam of laser-cooled atoms in an electric field.
2. Description of Related Art
Low emittance, high brightness charged particle beams suitable for focusing to the nanoscale have applications in a wide variety of areas. These areas include surface analysis, microscopy and surface modification.
A liquid metal ion source (LMIS) may be used for creating ion beams. Such beams are well-developed and perceived as reliable. When used in conjunction with an ion optical column, focused ion beams (FIBs) incorporating an LMIS may form a probe of a few nanometers in diameter. The focused probe may be formed when ions produced by the LMIS are accelerated to the desired energy and then focused onto a target. As the focused probe is rastered over the target secondary electrons, secondary ions or backscattered ions may be collected in order to form an image. The focused probe can also be used to modify a surface, for example, by removing material through sputtering. However, FIBs which employ these LMIS sources may offer larger spot size or lower currents than may be desirable for such applications.
There is a need for an ion source with higher brightness and lower energy spread than an LMIS.
More recently, charged particle sources based on photoionization of a confined, laser-cooled cold gas of atoms have become available. The magneto-optical trap ion source (MOTIS) is an example of such a charged particle source. When compared to LMIS-based FIBs, the MOTIS appears to offer some advantages. The MOTIS may be integrated with a commercial focused ion beam column, and may demonstrate beam currents and small spot sizes at e.g., 2 kilo-electron volts (keV) beam energy, which are substantially similar to LMIS-based FIBs. Also, the MOTIS-based FIB system may offer access to beams with atomic species not compatible with the LMIS. However, the MOTIS-based system may have a maximum brightness which is limited by diffusion of neutral atoms into the ionization volume.
Diffusion may not be an issue with some ion sources produced by laser-cooling. For example, a thermal source may have an on-axis oven that produces an unslowed atom beam. The output may be laser collimated, cooled along the two directions transverse to the atom beam's velocity, and then photoionized. Low chromatic spread may be possible in such a beam. This and subsequent proposals involve atom beams with large axial velocities and velocity spreads characteristic of thermal sources. They do not address major issues involved with efficiently converting an atom beam into a charged particle beam while achieving high beam brightness. Cooling atoms to very low temperatures in convenient distances and ionizing atoms efficiently with available laser power may be problematic in beams with large axial velocities due to the small interaction times with the corresponding lasers. Additionally, the coupling of energy between axial and transverse degrees of freedom in beams with a large axial velocity spread makes maintaining low transverse temperatures in the ion beam difficult.
Accordingly, in order to produce beams with brightness greater than the LMIS and suitable for demanding focused charged particle beam applications there is yet further a need for a new charged particle source which addresses the problems associated with previous on-axis oven-based designs. The charged particle source should still have a low energy spread.